An imaging device, such as an ink jet printer forms an image on a print media by ejecting ink from a printhead. Typically, an ink jet printhead includes at least one array of individually selectable ink jetting nozzles (e.g., a columnar array) that eject ink (such as by an actuator, such as a heater, piezoelectric device, MEMs device, or the like) to form a pattern of ink dots on the print media. The nozzles typically have a fixed vertical spacing between them, such as 1/600 inch. Additionally, the printhead (and therefore the array of nozzles) usually travels horizontally across the page (referred to hereinafter as a “scan”), with each nozzle being capable of ejecting ink at a fixed horizontal distance for a given scan speed, which can also be 1/600 inch, for example. The term “horizontal”, as used herein, indicates the direction of printhead travel, or scan, which is typically perpendicular to the vertical.
According to such an example, the vertical spacing of the nozzles, in combination with the horizontal distance between ink ejections, define a printing grid, or matrix, of pixels having a given vertical and horizontal resolution. The combined behavior of the scanning of the nozzle array and the amount of vertical movement (e.g., via indexing of the print media between consecutive scans) can allow, for example in some conventional printheads, ink to be ejected to form a dot at every pixel in a section of an image to be formed. In this condition, the corresponding grid of pixels is said to be “perfectly covered.” If the printhead can perfectly cover the grid in one scan, the printhead is said to operate in a perfectly covered print mode.
To accomplish a scan as previously described, an ink jet printer may include a reciprocating printhead carrier that transports one or more ink jet printheads across the print media along a (e.g., bi-directional) scanning path. A media advancing system is used to incrementally advance the print media in a media feed direction, also commonly referred to as a sub-scan direction or vertical direction, between scans in the main scan direction, or after all data intended to be printed with the print media at a particular stationary position has been completed. Also, typically, the columnar arrays of nozzles of the ink jet printhead, when mounted to the printhead carrier, extend in a direction parallel to the media feed direction.
For a given stationary position of the print media, printing may take place during one or more unidirectional scans of the printhead carrier. The term, unidirectional, often is used to refer to scanning in either, but only one, of the two bi-directional scanning directions. Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions.
The term, swath, refers to the area on the print medium traced by the printhead during a particular unidirectional scan of the printhead carrier where ink may be deposited. Thus, during the printing of a swath, individual printhead nozzles of the columnar nozzle array(s) trace along imaginary rasters spaced apart in the media feed direction and eject ink to form a printed pattern, such as for example printed lines, each line being formed by a plurality of ink dots. The swath height of a swath is determined, at least in part, by the extent of the columnar array of nozzles in the media feed direction, e.g., the distance between the top-most nozzle and the lower-most nozzle of the columnar nozzle array used in printing the swath.
Those working in the imaging arts continually strive to improve the print quality of imaging devices, such as ink jet printers. One such attempt is directed to reducing the occurrence of horizontal banding defects in images generated by an ink jet printer. Horizontal banding defects may be observed on print media, such as paper, as a horizontal white band. Such defects may be attributable to errors generated by the media advancing system that is used to advance the media in a media feed direction through the printer during the printing of the text or image on the media. Such errors can be caused, for example, by mechanical tolerances of an index roller and its associated drive train, if the same are used as part of the advancing system, and are often particularly noticeable at the boundaries of a swath.
It is known to mask such indexing errors by adopting an interlaced printing method, also referred to as shingling, wherein each scan of the printhead carrier (also sometimes referred to in the art as a printhead carriage), and therefore swath, is made to vertically overlap a preceding scan/swath. For a given swath, only a portion of the total print data for a given area on the print medium is printed (in other words, for example, only a portion of the pixels in the swath are addressable by ink dots). Thus, each scan of an actuated printhead produces a swath of printed output forming all or portions of multiple print lines, and multiple swaths may be required to complete the printing of any given print line.
Shingling is typically controlled by driver software/firmware associated with the printer (e.g., installed on a host computer, if such a host is present). The driver sends instructions to the printhead (e.g., via a printhead driver ASIC) regarding the specific dots to be placed on the given pass. One common shingling pattern is a 50% checkerboard pattern. For a 50% shingling mode (i.e., 2-pass or 50% interlace level), approximately 50% of the pixels are addressable with dots on any given pass of the printhead. The addressable pixels in a particular pass are typically selected according to a checkerboard pattern. The remaining 50% of the pixels necessary to form the desired images are addressed on a subsequent pass of the printhead. For a printhead with vertically disposed colors, and a raster which contains cyan, magenta and yellow dots, the printhead is usually passed at least six times with a 50% shingling method, twice depositing any cyan dots present in the given raster, twice depositing any magenta dots and twice depositing any yellow dots.
While the shingling method does reduce the line continuation errors of the ink jet printing, some line continuation errors are still noticeable. Furthermore, when a shingling method is employed, the printing speed can be greatly reduced due to the additional number of passes.
Modes of printer operation using a high number of printhead passes (e.g., 8, 16, 32, etc.), hereinafter referred to as high pass printer modes, such as those used in relatively higher quality operation modes (wherein the resolution may change), use shingling to mask swath boundary errors. In addition, the media advance system are often configured with an intentional underfeed, such as 0.3%, in an attempt to guarantee that there is some overlap between dots. Due to human visual perception, it is typically better to have a dark band due to the overlap rather than a white band.
Modes of printer operation wherein a low number of printhead passes are used (hereinafter referred to as low pass printer modes), such as those used in relatively lower quality operation modes, typically do not benefit as much from shingling (e.g., one pass printing inherently exhibits no benefit from shingling). For example, low pass printer modes are typically susceptible to errors in, for example, feedroll diameter and eccentricities causing excessive over or underfeed, swath expansion or compression caused by heater chip bow and rotary encoder error. All of these factors combined can have an effect on stitching together swath boundaries.
While low pass printer modes typically do not benefit from shingling, the print mode of the printhead (sometimes also referred to as the native printhead pattern) can effect, for example, swath overlap and other swath stitching errors. The print mode of a printhead refers to the pattern of pixels the printhead is capable of addressing during a single unidirectional scan (whether it actually forms dots on/in any given one of the pixels depends on, for example, the image being formed, whether shingling is being employed, etc.). As discussed above, in many conventional printheads, the printhead is configured to address each pixel (for a given colorant) in the section of an image corresponding to a respective swath in one pass.
Operating with this print mode, however, can cause problems, such as those resulting from ink drying issues and color image formation. To overcome this problem, it is known in the art to configure, and therefore operate, a printhead in a print mode in which only a fraction of the pixels in the section of the image corresponding to a swath can be addressed in one pass (wherein the unaddressable pixels can be addressed in one or more additional passes). One common printhead print mode is a checkerboard 50% print mode, wherein the printhead is configured to allow it to address, in a single scan, half of the pixels in the section of the image corresponding to that swath (for a given colarant), wherein the pixels addressable in that swath form a checkerboard pattern. Each printhead on conventional printers can typically only operate in one print mode. The print mode of the printhead may be subject to additional shingling print masks from the printer driver.
Various printhead print modes compensate differently for swath boundary stitching errors, drying issues and shrinkage of the print media. The effectiveness of printhead print modes can also be effected by the number of passes employed for the particular print job. As such, the inventors of the present invention have recognized that there is, for example, a need for a printhead capable of operating in an appropriate print mode for the number of passes to be used on a particular print job.